As noted in
https://github.com/llvm/llvm-project/pull/141821#issuecomment-2917328924,
whilst we currently constant fold intrinsics of fixed-length vectors via
their scalar counterpart, we don't do the same for scalable vectors.
This handles the scalable vector case when the operands are splats.
One weird snag in ConstantVector::getSplat was that it produced a undef
if passed in poison, so this also contains a fix by checking for
PoisonValue before UndefValue.
79ae407c95